It is well known that the surface of printing paper sheets is commonly coated with a paper coating formulation to improve the printability and to make it smooth and glossy.
Paper coating compositions generally comprise fillers or pigments dispersed in water, polymeric binders, rheology modifiers, water retention agents and dispersing agents (see by way of example “Handbook for Pulp & Paper Technology”, G. A. Smook, Angus Publications).
Dispersing agents are indispensable to reduce the viscosity in the presence of the high solid contents which are typical of paper coating compositions and to maintain a constant desired processing viscosity. Examples of conventional dispersing agents are complex phosphates, salts of polyphosphoric acid and salts of polycarboxylic acids.
Water retention agents prevent dewatering of the coating composition upon contact with the surface of the paper sheet. A typical water retention agent for paper coating composition is carboxymethyl cellulose.
Rheology modifiers are added to modulate the paper coating viscosity. Binders are responsible for the cohesion of the final coating and for its adhesion to the paper sheet.
Some conventional paper coating additives are known to perform more than one function; by way of example, carboxymethyl cellulose acts both as rheology modifier and water retention agent, whereas polyvinyl alcohol acts both as water retention agent and optical brightness enhancer.
WO 01/96007, WO 2004/044022, WO 2004/041883 and WO 2007/069037 describe the use of polyacrylic anionic copolymers in the paper industry, for making or coating paper; the polyacrylic anionic copolymers contain at least one anionic ethylenically unsaturated monomer having monocarboxylic functionality and at least one non-ionic ethylenically unsaturated monomer having poly(C2-4-alkylene oxide) functionalities. They are said to be useful as dispersing and/or grinding agents, as agents for improving the optical brightening activation, as water retention agents, as viscosity and gloss enhancers. Unfortunately, when the known ionic substances are used in aqueous dispersions in accordance with the prior art, their effectiveness is dependent on the pH value of the dispersion.
It has now been found that specific polyethers containing polyoxyethylene chains, some of which have not been previously described in the literature, are suitable as deflocculants, water retention agents and gloss enhancer for paper coating compositions; the resulting paper coating compositions are stable over a wide viscosity range irrespective of their pH value and impart good printability and brightness to the coated paper.
The polyethers containing polyoxyethylene chains are water soluble and can be used for the preparation of paper coating compositions in the form of aqueous solution.